1. Field of the Invention
The present invention relates to hydraulic steering systems for aircraft landing gear wheels, and more particularly to mechanisms for damping vibration in such steering systems.
2. Description of the Related Art
Landing gear assemblies that are retractable into the fuselage of an aircraft include an actuator for raising and lowering the landing gear with respect to the fuselage and another actuator for turning the nose landing gear to steer the aircraft during taxiing. One common type of steering actuator utilizes a hydraulic motor connected by a gear box to a vertical strut on which the aircraft wheels are mounted. By applying pressurized fluid to different ports of the hydraulic motor, the strut can be rotated in either of two directions about a vertical axis to turn the landing gear wheels left or right. A servo valve assembly controls the flow of hydraulic fluid through the motor between a supply line from a pump and a tank return line leading back to a reservoir for the hydraulic system. Different positions of the valve assembly determine into which motor port the pressurized fluid is directed and thus the direction of the turning motion. The valve assembly is operated in response to manipulation of an steering control mechanism by the pilot.
Some aircraft nose wheel configurations are dynamically unstable which may result in the wheel oscillating about its caster axis, commonly known as shimmy. A damping mechanism is frequently provided to reduce that shimmy when power is not being applied to the steering motor. In this “free caster” mode, the ports of the hydraulic motor are disconnected from the supply and return lines and a damping orifice is connected between the ports. Thus, the flow of fluid between the two ports is restricted thereby damping motion of the nose wheel about the caster axis.
However, when the hydraulic motor is being powered, a shimmy damping orifice provides a path in parallel with the motor through which fluid flows from the supply line directly to the tank return line. The damping orifice thus provides a leakage path for the pressurized fluid to by-pass the hydraulic motor, thereby resulting in an energy loss.
As a consequence, it is desirable to provide a shimmy damping mechanism that does not result in an appreciable energy loss.